Analysis on the Optical Response of Ldr and Phototransistor in Photoplethysmography

Abstract

In photoplethysmography, light source and light detector are employed such that the received signal corresponds to a particular information needed. This research used five different LEDs as light sources. Phototransistor and LDR, on the other hand, were used as light detectors to assess the photosensitivity of human skin. Backscatter simulation was used to compare the photoplethysmographic detection of LDR and phototransistor. The aims of this study include: determine the optical response of a human skin, particularly the skin on the dorsal and palmar parts of the hand; conduct simultaneous simulation using modular and backscattered setup; provide measured value based on the conducted simulations. In every LED color and for both light detectors, ten test conditions were performed, namely: (1) room light on; (2) room light off; (3) room light on - black paper; (4) room light off - black paper; (5) room light on - white paper; (6) room light off – white paper; (7) room light on – dorsal skin; (8) room light off – dorsal skin; (9) room light on – palmar skin; and (10) room light off – palmar skin. Two different simulations were conducted involving all test conditions. The second simulation used light meter which showed similarity in behaviour of obtained values vis-à-vis barcode meter and ADC voltage. The modular involved a lux sensor which checked the findings on the reflectivity of skin Types III and IV of the Fitzpatrick scale and melanocytic naevi skin. The series of simulations conducted proved that the yellow LED provided highest sensor response and phototransistor greater sensitivity than LDR in detecting the received signals. Room light off condition was necessary to reduce the effects of ambient light.